Channel Stabilization with Basic Flow Calculations

Transcription

1 Channel Stabilization with Basic Flow Calculations Description Selection Criteria The selection of a channel lining will greatly influence how a drainage channel performs, the amount of erosion and scour, the frequency and cost of maintenance, appearance, aesthetics, and even safety. In addition, the amount of sediment and nutrients conveyed can be influenced greatly by the type of channel lining selected. This BMP examines different factors and some basic design parameters for channels and channel linings. Suitable applications are any areas which regularly receive and convey concentrated stormwater runoff, such as drainage channels, ditches, or swales. Channel linings can also be used in areas which occasionally convey stormwater runoff, such as overland relief swales or emergency spillways. Every drainage channel, ditch, or swale must have some type of channel lining. By default and if not specified, then the existing channel lining is native soil or rock. The least expensive and most beneficial lining is usually a grass channel if design parameters do not indicate excessive velocities, regular submergence, inadequate flow capacity, or potential maintenance problems. Grass channels are easy to maintain, flexible and self-healing, attractive in appearance, remove pollutants (see 9.5 Filter Strips and Swales), and decrease the amount of runoff by allowing stormwater infiltration and evapotranspiration. Grass channels are an example of a flexible lining (may also be called a soft or green lining) which include vegetation as the principal means of preventing erosion. A variety of temporary and permanent geosynthetic products can help to establish a soft lining; common examples are erosion control matting, excelsior blankets, geogrids filled with soil, or turf reinforcement mats. Soft linings are aesthetically pleasing, flexible, and easy to install and maintain. Some drawbacks to soft linings are the potential for damage by heavy traffic, excessive drought or pollution. Riprap and concrete and some geosynthetic channel linings are examples of rigid or hard linings. These channel linings are used when design velocities exceed permissible values for soft linings, or to improve flow capacity by reducing roughness and flow losses. Hard linings must be installed in a controlled manner with proper materials, compaction, bedding, and anchoring in order to prevent scour, undercutting or settlement. By law, anyone who works within or along a stream must obtain an Aquatic Resource Alteration Permit (ARAP) from the TDEC Division of Water Pollution Control. The ARAP is required for activities such as: dredging, widening a stream channel, straightening a stream channel, building a dock or boat ramp, altering a wetland, utility line crossings or streambank stabilization. Visit the TDEC permitting website for more information at Page 1 of 10

2 Best Management Practices Manual Related BMPS which impact the selection of channel linings include: Matting (MA) Geotextile (GE) Diversion Ditch (DI) Riprap (RR) Slope Drain (SD ) Basic Flow Calculations This section contains a description of basic flow computations for use in designing an open channel, ditch or swale. Drainage channels and ditches should generally be designed by a professional engineer to ensure that adequate drainage capacity and allowable flow velocities are provided. Open-channel computations are usually in the form of Manning s equation: V = (1.49 / n ) R H 2/3 S 1/2, where V = average velocity in channel (feet per second) n = Manning s roughness coefficient (dimensionless) R H = hydraulic radius of channel = A / W P (expressed in feet) S = energy grade line = channel slope for uniform flow (dimensionless) A = cross-sectional flow area (square feet) W P = wetted perimeter of flow (feet) The total flow through the channel (Q, expressed in cubic feet per second) is equal to the velocity times the cross-sectional flow area: Q = V A Figure 1 Basic Flow Computation Manning s Equation 1.21 sq ft area = 0.5 x 1.1 x feet W P = ( ) 0.5 B = 4 feet 1.82 sq ft area 3.48 feet W P Given: D = 1.10 feet n = S = 0.01 ft / ft 2:1 slope 4.40 sq ft area 4.00 feet W P 3:1 slope channel geometry Computed: A = 7.43 sq ft W P = 9.94 feet R H = feet V = 4.91 fps Q = 36.5 cfs Page 2 of 10

3 Chattanooga Hamilton County Area Water Quality Programs Manning s equation is for open-channel flow and assumes a constant uniform flow rate at a specified slope. There are many factors which can affect this assumption, such as varying channel widths and slopes, downstream flow restrictions, backwater from dams or other berms, culvert entrance and exit losses, headwater at culverts or bridges, channel bends, varying lining materials, etc. Any of these factors will generally require that an experienced professional engineer be responsible for design and analysis. In addition, channels with unusual shapes, composite materials or uneven sections will generally require that a professional engineer with knowledge and experience should be responsible for the design and analysis. In addition, channels with unusual shapes, composite materials or uneven sections will generally require that a professional engineer with knowledge and experience should be responsible for the design and analysis. The major difficulty in estimating velocity and flow is usually the selection of Manning s roughness coefficient n. Typical values are listed in Table 1 and Table 2. See Table 3 for n values of grass channels, based upon type and height of vegetation, and product of velocity (V) and hydraulic radius (R H ). Subcritical and Supercritical Flow It is useful to know whether a flow is subcritical (also called tranquil flow, backwater flow or downstream control) or supercritical (also called rapid flow or upstream control). This is determined by computing the Froude number; a value of F R less than 1 is subcritical and a value greater than 1 is supercritical. Subcritical flow is greatly preferred because it has a lower velocity than supercritical flow. A value of F R between 0.8 and 1.2 indicates that the channel is close to critical flow, and that small changes in channel cross section, flows, slopes, etc., may cause the water surface to change radically or even create a hydraulic jump or standing wave. Open channels should not be designed at or near critical flow conditions. F R = ( ( Q 2 * T) / (g * A 3 ) ) 1/2, where F R = Froude number (dimensionless) Q = discharge or flow (cubic feet per second) T = top width of water surface (feet) g = gravitational constant = 32.2 feet/ second 2 A = cross-sectional flow area (square feet) (for Figure 1) F R = ( ( * 9.5) / (32.2 * ) ) 0.5 = 0.98 The example channel in Figure 1 is approximately at critical flow and should be changed. Since subcritical flow is the preferred flow regime, this can be accomplished by widening the channel, flattening the side slopes, increasing the Manning s roughness coefficient n, or decreasing the channel slope. Critical depth (D C ) indicates the flow depth for which the specific energy (E) is at a minimum value for a given discharge. Specific energy is computed by the equation: E = D + V2 / (2g) For the example in Figure 1, the specific energy is feet. Using the example in Figure 1, a roughness coefficient value of corresponds to any of several channel linings in Table 1 such as: Bare earth, straight and uniform, short grass Erosion control matting (excelsior mat) Rocky channel, smooth and very uniform Page 3 of 10

4 Best Management Practices Manual Using same geometry as shown in Figure 1 with a grass channel lining instead will yield the following two sets of answers for the same given flow of 36.5 cfs. In general, a conservative design will use unmowed grass to check conveyance and mowed grass to check for velocities. So the design depth would be 2.06 feet and the design velocity would be 3.56 fps. Table 1 Manning s Roughness Coefficient Channels Closed Conduits n Brick Cast-iron pipe Cemented rubble Concrete pipe Corrugated metal pipe, plain, regular corrugations Corrugated metal pipe, asphalt-paved invert, flowing full Closed Conduits n Corrugated metal pipe, asphalt-paved, 50% flow depth Corrugate metal pipe, large corrugations (1 or 2 deep) Plastic pipe, smooth/corrugated (consult manufacturer) PVC pipe Steel pipe Vitrified clay Open Channels n Asphalt pavement Bare earth, straight and uniform, no vegetation Bare earth, straight and uniform, with some short grass Bare earth, winding and sluggish Bare earth, winding and sluggish, with some short grass Brick Cemented rubble Concrete channel, unfinished Concrete channel, troweled Concrete channel, troweled with exposed gravel finish Concrete channel with mortared or riprap sides Concrete gutter, finished and troweled Erosion control matting (excelsior mat or straw netting) Erosion control matting (jute net) Grass Table 3 Gravel or aggregate, compacted Gravel bottom, with weeds on banks Riprap, dumped (n chosen from D50 size) See 5.8 Riprap Riprap, grouted and placed as a smooth uniform channel Rocky channel, smooth and uniform Rocky channel, irregular and winding Weeds and brush, uncut, only on banks Weeds and brush, uncut, across entire channel Page 4 of 10

5 Chattanooga Hamilton County Area Water Quality Programs Grass channels are frequently grouped into categories based upon the retardance that reflects the height and type of vegetation, flow characteristics of channel, etc. The retardance classification taken from Table 3 is then used in Figure 2 to select a Manning s roughness coeffficient based upon the product of velocity, V, and the hydraulic radius, R H. Solving Manning s equation for a grass surface, due to the variable roughness coefficient, is an iterative process for which a spreadsheet may be helpful. Table 2 Manning s Roughness Coefficient Natural Channels Closed Conduits Natural Stream (less than 100 feet wide at flood stage) Clean, straight, no rifts or deep pools, grass banks* Clean, straight, grass with some stones and weeds* Clean, winding, pools and shoals* Clean, winding, pools and shoals, some stones and weeds* * Values may be increased by the largest of the 4 possible adjustments below: 1. Adjust upward by for lower stages or ineffective flow areas 2. Adjust upward by for larger stone and weeds 3. Adjust upward by to for partially submerged trees / branches 4. Adjust upward by to for entire submerged trees in channel Sluggish reaches, deep pools, many weeds Sluggish, many deep pools, full of weeds, heavy timber Mountain stream, gravel and cobbles, with steep banks Mountain stream, cobbles and boulders, with steep banks In general, n values are lower for larger streams because the banks offer less resistance. Usually larger streams have been modeled by government agencies such as TVA, FEMA, or the City of Chattanooga so that some guidance is available on roughness coefficients used. Manning s n values can be substantially different during summer when vegetation may be overgrown and trees contain branches full of leaves. Adjust values upward if stream flow submerges trees and tree branches. Floodplains (adjacent to natural streams) Cleared land with tree stumps Pasture, no brush, short grass Pasture, no brush, high grass Farmland, no crops Farmland, mature crops Heavy weeds, scattered brush Light brush and trees Medium to dense brush Dense brush, thick trees, undergrowth, fallen logs n n Page 5 of 10

6 Best Management Practices Manual Natural streams have constantly varying cross sections and slopes, so that the Manning s equation should be used carefully with the understanding that other factors may affect flow depth. Therefore, the use of Manning s equation for natural streams should only be for rough estimating purposes. Water surface profile programs (such as HEC-2 and HEC-RAS developed by the US Army Corps of Engineers Hydraulic Engineering Center and WSPRO) can handle multiple roughness coefficients, complex geometry, bridges, culverts, flow obstructions, and varied flow values into consideration. Water surface profiles must be prepared by a professional engineer using the best available data. Table 3 Retardance Classifications for Grass Channels Class Type of Vegetation Condition Yellow bluestem ischaemum A Weeping lovegrass Alfalfa Bermudagrass Blue gamma Kudzu B Reed canarygrass Sericea lespedeza Tall fescue Weeping lovegrass Grass mixture #1 Grass mixture #2 Bahiagrass Bermudagrass Centipedegrass C Crabgrass Kentucky bluegrass Redtop Tall fescue Grass mixture #3 Bahiagrass D Bermudagrass Buffalograss Centipedegrass Kentucky bluegrass Red fescue Sericea lespedeza Tall fescue Grass mixture #4 E Bermudagrass Any type of grass Native grass mixture #1 Summer grass mixture #2 Summer grass mixture #3 Excellent stand, tall, 36 average Excellent stand, tall, 30 average Good stand, uncut, 11 Good stand, tall, 12 Good stand, uncut, 13 Very dense growth, uncut Good stand, cut, 12 to 15 Good stand, not woody, tall, 19 Good stand, uncut, 18 Good stand, uncut, 13 Good stand, uncut Good stand, uncut, 20 Good stand, uncut, 6 to 8 Good stand, cut, 6 to 8 Very dense cover, 6 to 8 Fair stand, uncut, 10 and longer Good stand, headed, 8 to 10 Good stand, uncut, 15 to 20 Good stand, cut or uncut, 6 to 8 Good stand, uncut, 6 to 8 Good stand, cut, 3 to 4 Good stand, cut, 2.5 Good stand, uncut, 3 to 6 Good stand, cut, 3 to 4 Good stand, cut, 3 to 4 Good stand, uncut, 12 Good stand, cut, 2 Good stand, cut, 3 to 4 Good stand, uncut, 4 to 5 Good stand, cut, 1.5 Burned or trampled, any length - prairie grasses, bluestem, blue gamma - tall fescue, red fescue, sericea lespedeza - timothygrass, smooth bromegrass or orchardgrass Spring/autumn grass mixture #4 - orchardgrass, redtop, annual lespedeza Page 6 of 10

7 Chattanooga Hamilton County Area Water Quality Programs Grass channels are often designed as a parabolic shape without any corners or slope breaks. The following formulas for cross-sectional flow area (A) and hydraulic radius (R H ) are based on the top width of flow (T) and maximum flow depth at the center of channel (D): A = 2/3 (T D) R H = ( T 2 D) / (1.5 T D 2 ) UNMOWED Retardance B: Q = 36.5 cfs n = D = 2.06 feet A = sq ft W P = feet R H = feet V = 1.94 fps MOWED Retardance D: Q = 36.5 cfs n = D = 1.38 feet A = sq ft W P = feet R H = feet V = 3.56 fps Figure 2 Manning s Roughness Coefficient Grass Channels D E C B A V R H (product of velocity and hydraulic radius) Page 7 of 10

8 Best Management Practices Manual A channel lining may be judged adequate or permissible based on two possible criteria, either permissible shear stress or permissible velocity. Permissible shear stress is based on the force necessary to displace or move the soil, aggregate, or other type of channel lining. The formula for normal shear stress (T) at the bottom of a uniform channel is shown below. This value is adjusted for several factors such as side slope, bend angles, shape of channel, etc., before being compared to published values of permissible shear stress. T = γ D S T = shear stress (pounds per square foot) γ = unit weight of water (62.4 pounds per cubic foot) D = flow depth of water (feet) S = channel slope (feet per foot) The simpler design method is to specify a permissible velocity for each type of channel lining. Typical permissible velocities are listed in Table 4. In general, a temporary channel lining should be considered if the design flow velocity for bare soil is greater than 2 feet per second. For preliminary design, a soil may be considered erodible if it has a published K value of 0.35 or greater in the Hamilton County soils map. Table 4 Permissible Velocities Channel Lining Material Permissible Velocity (fps) Silt or very fine-grained materials 1.5 Fine sand, sandy loam, silty loam 2.0 Undisturbed alluvial sediments 3.5 Stiff clay 3.5 Coarse sand or fine gravel (no silt) 4.0 Coarse gravel 5.0 Cobbles, hard pan, shale 5.5 Erodible Soil (silt, loam, sand) 0-5% 5-10% >10% Bermuda grass Bahia grass, Blue Gamma, Kentucky bluegrass Reed canary grass, Tall fescue Mixture (fescue, lespedeza., legumes) Alfalfa, Crabgrass, Kudzu, Sericea lespedeza Weeping love grass, Yellow bluestem Resistant Soil (gravel, clay, cohesive) Bermuda grass Bahia grass, Blue Gamma, Kentucky bluegrass Reed canarygrass, Tall fescue Mixture (fescue, lespedeza., legumes) Alfalfa, Crabgrass, Kudzu, Sericea lespedeza Weeping love grass, Yellow bluestem Page 8 of 10

9 Chattanooga Hamilton County Area Water Quality Programs Maintenance Limitations References Channel linings should be inspected at least weekly during the construction phases to ensure proper functioning and necessary control of erosion and sediment. Inspect channels monthly during the first year after construction to verify that drainage channels work properly as designed and constructed. After the first year, channel linings should be inspected at least quarterly on a permanent basis. Look for erosion, siltation, undercutting or settlement throughout the length of channel. Verify that upstream and downstream portions of channel are not adversely affected. Flexible channel linings need frequent maintenance and inspections to ensure adequate function and erosion control. Soft channel linings can be damaged or stressed due to many factors. Rigid permanent channel linings often result in prevention of habitat establishment. Hard linings may be damaged due to settlement, scour or undercutting despite the best efforts and care taken during installation. Inadequate coverage or depth of channel linings will result in erosion, washout, and poor plant establishment. If the channel grade and liner are not appropriate for the amount of runoff, channel bottom erosion may result. Riprap must be sized correctly and installed according to correct procedures. If the channel slope is too steep or riprap is too small, displacement may occur. Displaced riprap may obstruct channel or cause additional damage (see Section 5.8 Riprap). California Department of Transportation Caltrans Highway Design Manual. Charlotte (City of), and Mecklenburg County. July Charlotte Mecklenburg Storm Water Design Manual. Georgia State Soil and Water Conservation Commission (SSWCC) Manual for Erosion and Sediment Control in Georgia. Fifth Edition. Kentucky Transportation Cabinet. January Drainage Guidance Manual. North Carolina Sediment Control Commission. September Erosion and Sediment Control Planning and Design Manual. Smoot, James and Russell Smith. December Soil Erosion Prevention & Sediment Control. University of Tennessee, Department of Civil & Environmental Engineering. United States Bureau of Public Roads. August Design Charts for Open-Channel Flow, Hydraulic Design Series No. 3. United States Bureau of Public Roads. May Design of Roadside Drainage Channels, Hydraulic Design Series No. 4. University of Tennessee Department of Civil and Environmental Engineering. August Soil Erosion Prevention and Sediment Control - Reducing Nonpoint Source Water Pollution on Construction Sites. Virginia, Department of Conservation and Recreation, Division of Soil and Water Conservation Virginia Erosion and Sedimentation Control Handbook, Third Edition. Page 9 of 10

10 Best Management Practices Manual Photos 1-2 An Application of Soft or Green Channel Lining Page 10 of 10